Ball grid array ("BGA") and land grid array ("LGA") are technologies presently used for mounting packaged semiconductor components onto boards. A semiconductor component is packaged by mounting it on a plastic substrate. The component is coupled to leads on the substrate, either by wirebonding or using solderbumps to electrically connect bond pads of the component onto the substrate. The component is protected from the ambient by an encapsulant that covers the entire top surface of the component and a portion of the substrate on which the component is mounted. The underside of the plastic substrate contains a two-dimensional array of contacts. If the package is BGA, then the contacts contain solder balls. If the package is LGA, the contacts contain lands. The contacts are arrayed in a matrix, the individual contacts being spaced approximately 1 millimeter apart, starting at approximately 1.5 millimeters inside bottom surface from the edge of the substrate.
In some applications of socketing a BGA and LGA component, an interposer with pins is used to provide an interface between the component and the motherboard. The interposer with pins may be useful if the user desires to plug the component into a conventional pin socket connection in a motherboard, as supported by electronics industry standards. The BGA or LGA may also be direct-socketed, that is, without using an interposer. If the component is direct-socketed, it is often necessary to use a stiffener on the underside of the motherboard opposite the side of the BGA or LGA component. Moreover, the motherboard usually must include gold plating on the motherboard contacts to enable acceptable electrical coupling between the BGA or LGA component and motherboard pursuant to industry requirements.
A present configuration for socketing a BGA or LGA component onto a motherboard without using an interposer is shown in side view, in FIG. 1. A motherboard 10 is provided. Motherboard 10 contains gold plating (not shown) to serve as the electrical contact points for a BGA or LGA component 20. A spacer 15 is provided directly above motherboard 10. Component 20 is provided above motherboard 10 and is surrounded by spacer 15. A compressible spring interconnect 25 made of elastomer or spring contact is disposed between component 20 and motherboard 10 and helps to improve electrical contact between component 20 and motherboard 10. A lid 35 is placed directly above component 20 to protect component 20 and to provide an interface to a heat sink if needed. A board stiffener 30 is disposed on the side of motherboard 10 opposite the surface that is coupled to component 20. Board stiffener 30 is necessary to spread a normal force of about 50 grams per contact, on each contact, from component 20. Without board stiffener 30, motherboard 10 will deflect from the force exerted from compressible spring interconnect 25. Board 30 stiffener contains tapped holes 40. Motherboard 10 contains screw holes 45 in alignment with stiffener tapped holes 40. Spacer 15 also contains screw holes 50 in alignment with board tapped holes 40. Lid 35 contains screw holes 55 in alignment with spacer screw holes 50. Lid 35, spacer 15, motherboard 10, and stiffener 30 are all clamped together with screws 60.
A problem with the present method of direct socketing is in the use of a stiffener on the backside of the motherboard. Having a stiffener adds significant cost to manufacturing due to the necessary additional materials. The stiffener also consumes real estate on the backside of the motherboard which could otherwise be used for placement of active components. Attaching the stiffener using screws requires additional manual labor, which adds further cost. In addition, to make a direct socketing solution viable, gold plating must be done on the motherboard contacts due to inadequacies with solder ball material and solder contacts in providing the necessary degree of electrical coupling. Adding gold plating to a motherboard adds cost to the board which is undesirable.
An alternative scheme for connecting a component to a motherboard is to use an interposer. This may be desirable for providing a socket for an electrical component that is consistent with normally used pin grid array connectors in the electronics industry. An example configuration of a BGA component utilizing an interposer for attachment to the motherboard is shown in FIGS. 2A-2B. A motherboard 80 holds a socketing stage 85. Socketing stage 85 is coupled to motherboard 80 using solder balls 90 on its underside, in a ball grid array. Solder balls 90 provide electrical contact between socketing stage 85 and motherboard 80. Holes 95 extending through the top surface of socketing stage 85 allow socketing stage 85 to serve as a recipient for a pin grid array interposer 100. Interposer 100, functioning as a normal pin-grid array socket, contains a series of pins 105 which are matched in location with corresponding holes 95 of socketing stage 85 so that when interposer 100 is inserted into socketing stage 85, pins 105 come into electrical contact with stage 85 and are thereby coupled to motherboard 80. The top surface of interposer 100 contains lands (not shown) so that interposer 100 can be electrically coupled to a BGA component 110 sitting above. BGA component 110 is shown to be disposed above interposer 100. Solder balls 120 on BGA component 110 are matched with corresponding lands (not shown) on interposer 100 so that BGA component 110 comes into electrical contact with interposer 100. The combination of interposer 100 with accompanying BGA component 110 forms the piggyback component 130 that can be inserted into socketing stage 85 as shown in FIG. 2B. By inserting piggyback component 130 in and out of socketing stage 85, the solder balls 90 on socketing stage 85 can remain intact and not be broken in the event component 110 needs to be exchanged with another component. Having socketing stage 85 remain stationary on motherboard 80, and the ability to socket items into and out of socketing stage 85 is desirable particularly for testing the reliability of motherboard 80 or components that may be coupled to motherboard 80.
While the interposer approach described here may be useful for testing, it is not desirable for production components due to the cost of the needed additional materials. Note that the interposer approach utilizes two substrates between the component and the motherboard. The substrates add cost to manufacturing by requiring extra materials.
It would be advantageous to provide a method of socketing a BGA or LGA component into the motherboard without having to use a stiffener, gold plating on the motherboard, or an interposer.